The present invention relates to a sunscreen cosmetic containing a micro particle zinc oxide, especially to the improvement thereof in the stability and the ultraviolet rays protection effect.
Conventionally, a sunscreen cosmetic contains a large amount of a micro particle zinc oxide for the purpose of protecting skin from a long-waved ultraviolet light. In a case where a micro particle zinc oxide is added mainly to a sunscreen cosmetic of a W/O emulsion type, a micro particle zinc oxide whose particulate surface is imparted with a hydrophobicity has generally been employed. Such hydrophobicity-imparting treatment is effected for example with a silicone, a metallic soap, a fatty acid, a fluoride and the like.
However, a highly dispersed state of such surface-treated micro particle zinc oxide is achieved only with an extreme difficulty, due to which a satisfactory ultraviolet rays protection effect can not actually be obtained. Such highly dispersed state, when accomplished using a media-agitating mill or a high pressure dispersing machine, results in an improvement in the ultraviolet rays protection effect. Nevertheless, a surface treatment coating may be peeled off to expose a non-treated hydrophilic surface which leads to an aggregation of a particle in an aqueous phase as well as a susceptibility to a reaction with a free fatty acid present in the system to form a gel, resulting in an extremely poor stability during a prolonged storage period.
The present invention is established in view of the problems discussed above, and thus its objective is to provide a sunscreen cosmetic in which a micro particle zinc oxide is dispersed satisfactorily and which is excellent in the ultraviolet rays protection effect as indicated by SPF (sun protection factor) and PFA (protection factor of UVA), and also in the finish transparency and the system stability.
The inventors discovered that a composite powder obtained by treating the surface of a micro particle zinc oxide in a form close to a primary particle with a silicic anhydride followed by performing a silicone treatment can readily and highly be dispersed by a polyoxyalkylene-modified polysiloxane. Also, the inventors discovered that this is associated also with an efficient improvement in SPF and PFA of a sunscreen cosmetic. Furthermore, this composite powder undergoes no reaction with a free fatty acid in the system (a gelation reaction due to the formation of a metallic soap) resulted from the presence of a fatty acid or a fatty acid soap employed as a particle dispersing agent or a surface treating agent. Accordingly, it was discovered that the viscosity of the system was kept stably to give a use feeling which was maintained in a favorable condition.
A silicone-treated composite powder described above undergoes no reaction with water due to the exposure of a non-treated surface even when subjected to a strong mechanical force such as those exerted by a media-agitating mill or a high pressure dispersing machine. In addition, this composite powder undergoes no reaction with a free fatty acid in the system due to a fatty acid or a fatty acid soap which is employed as a dispersing agent or a surface treating agent for other powders. Accordingly, it was discovered that a favorable use feeling was maintained. Furthermore, it discovered that a highly dispersed powder served to improve SPF and PFA of a sunscreen cosmetic efficiently.
Namely, in the first aspect of the present invention, a sunscreen cosmetic is obtained by means of an incorporation of a composite powder of a micro particle zinc oxide which is coated with a silicic anhydride and whose surface is treated with a silicone, a dispersing treatment of said composite powder in an oil or water using a media-agitating mill or a high pressure dispersing machine and/or an incorporation of a polyoxyalkylene-modified polysiloxane.
In a sunscreen cosmetic of the present invention, it is preferable that said polyoxyalkylene-modified polysiloxane is one or more selected from those represented by general formulae [1], [2] and [3]: 
(in general formulae [1], [2] and [3], R denotes a methyl group or partly a phenyl group, R denotes hydrogen or an alkyl group having 1 to 12 carbon atoms, p denotes an integer of 1 to 5, q denotes an integer of 2 to 3, and x, m and n denote mean numbers giving a polyoxyalkylene-modified organopolysiloxane which contains in its molecule 5 to 40 wt % of the polyoxyalkylene group and whose molecular weight is 2000 or more.)
Also in a sunscreen cosmetic of the present invention, it is preferable that said composite powder is a composite powder obtained by coating a microparticulate zinc oxide having a particle size of 0.1 xcexcm or less with a silicic anhydride at a coating rate of 5 to 30 wt % followed by a surface treatment with 3 to 12 wt % of a silicone.
Also in a sunscreen cosmetic of the present invention, it is preferable that said composite powder of a micro particle zinc oxide treated with a silicic anhydride and a silicone is incorporated in an amount of 3 to 30 wt % and said polyoxyalkylene-modified polysiloxane is incorporated in an amount of 0.5 to 7 wt %.
In the second aspect of a sunscreen cosmetic of the present invention, 1 wt % or more of a powder whose surface is treated with one or more substances selected from fatty acids, fatty acid soaps and fatty acid esters is further contained.
In the third aspect of a sunscreen cosmetic of the present invention, 0.1 wt % or more of one or more substances selected from fatty acids, fatty acid soaps and fatty acid esters are further incorporated as a surface treating agent for a powder.